The field of the present invention is the design, fabrication, and manufacture of gallium arsenide (GaAs) integrated circuits. More particularly, the invention relates to an integrated circuit employing Gallium Arsenide (GaAs) material and processes, and the process of attaching the integrated circuit to a circuit device.
Modern electronics equipment often needs efficient power transistors with good radio frequency characteristics. For example, wireless devices typically have a radio and associated circuitry generating a low-level radio frequency signal. This low-level radio frequency signal needs to be amplified for transmission from an antenna system. The use and manufacture of power transistors are well-known, and have advanced to create highly efficient and effective power transistors. For example, power transistors may be made using a GaAs (Gallium Arsenide) material and process. The GaAs material and process have been found to create power transistors with particularly desirable radio frequency characteristics, have high yields, and be cost competitive with other technologies due to their high power densities.
In one example of a GaAs integrated circuit, a GaAs power transistor is formed by first depositing epitaxial layers on the GaAs substrate and then etching to the appropriate layers, followed by depositing a metal contact for terminals of the device. The GaAs power transistor is typically grown on a semi-insulating substrate on which a GaAs contact layer (sub-collector) is deposited. This sub-collector layer is on the contact side of the integrated circuit, and cooperates with a gold contact layer to attach to a pad on a printed circuit board, for example. A GaAs collector region is deposited on top of the sub-collector layer. The base layer is then deposited atop the collector. Next, the emitter layer (a wide bandgap semiconductor) is deposited. On top of this emitter layer, an emitter contacting layer, so that contact resistance can be minimized, is finally deposited. After the growth of the material, the emitter, base and collector contacts are formed by etching to the specific layers and depositing contact metals.
The use of gallium arsenide substrates in the design and construction of integrated circuits has proven to have desirable effects. For example, gallium arsenide substrates have been useful in achieving greater performance in power amplifier integrated circuits, for example. Typically, a gallium arsenide integrated circuit will be used as a component in a larger circuit device or design. In order to be integrated into the circuit design, the gallium arsenide integrated circuit is typically mechanically and electrically coupled to a printed circuit board for the circuit device. In other cases, the gallium arsenide integrated device is mounted to other electronic devices.
A typical gallium arsenide integrated circuit has a gallium arsenide substrate having a set of deposited materials cooperating to implement a particular circuit function. Often, the circuit side is coupled to other device components using wire bond or pin technology. The contact side of the gallium arsenide integrated circuit is typically adhered to a large contact pad on the device's printed circuit board. More particularly, the integrated circuit has a gold contact layer which adheres to the printed circuit board pad using a conductive adhesive. Since the conductive adhesive is intended to flow when under pressure, an amount of the conductive adhesive escapes from beneath the contact as the contact is pressed to the printed circuit board pad. In its final arrangement, a layer of between about 30 to 40 micron of conductive adhesive rests between the gold contact and the printed circuit board pad, and an additional quantity of the conductive adhesive sits as excess in a reflow area around the contact. Accordingly, to accommodate this expected reflow material, the printed circuit board pad is made considerably larger than the actual gold contact on the gallium arsenide integrated circuit.
The gallium arsenide integrated circuit typically has a gallium arsenide substrate with a titanium tungsten (TiW) layer on its contact side. This titanium tungsten (TiW) layer may be approximately 500 angstroms thick, and is useful for improving the application of the gold contact. A layer of gold contact material is deposited on to the titanium tungsten (TiW) at a thickness of about 5 micron. Often, the gallium arsenide substrate has vias which extend into or through the substrate for facilitating electrical flow vertically through the substrate. These vias are also coated with the gold conductive material. However, gold deposits in a non-uniform manner, causing areas of relative thick and relative thin coatings on the walls. This non-uniformity not only has detrimental electrical effects, but also results in using excess gold material, which increases the cost of making the GaAs integrated circuit.
An electroplating process is typically used to deposit the gold material for the gold contact and in the vias. However, the electroplating of gold is typically done at about a 25% duty cycle. This means that for each one hour of time in the electroplating bath, gold is only being deposited for about 15 minutes. In this way, the depositing of gold layer material is a time-consuming and relatively inefficient process. Also, gold is an expensive material, increasing the cost for gallium arsenide integrated circuit products. Finally, gold has a relatively high dissolution rate in solder, and therefore is not able to be soldered to the pad of the device's printed circuit board. Instead, conductive adhesive is typically used to adhere the gold contact to the printed circuit board pad. The use of conductive adhesive requires an additional manufacturing step, and also requires the use of larger pads to accommodate adhesive overflow. However, even with these undesirable features, gold continues to be the standard metal used for contact layer on a gallium arsenide integrated circuit.
Other integrated circuit technologies, such as silicon based technologies, use copper for its contact layer. Copper has superior conductivity, may be applied more uniformly, and is a less costly material. Further, copper has a sufficiently low dissolution rate in solder, so allows the integrated circuit device to be soldered to its printed circuit board pad. However, copper readily oxidizes, which degrades electrical and mechanical characteristics. Accordingly, when used in the silicon process, the copper is applied in thick layers and polished and capped with dielectric materials such as silicon nitride to avoid these oxidation effects.
Although copper has been successfully used in silicon wafer technology, it has not been successfully used in gallium arsenide integrated circuit devices. Copper could readily pass through very thin titanium tungsten (TiW) at high temperatures and diffuse into the gallium arsenide substrate. This diffusion greatly interferes with the electrical characteristics of the gallium arsenide based devices, causing the gallium arsenide integrated circuit to fail or function improperly. Accordingly, the use of copper results in the destruction or nonoperation for gallium arsenide integrated circuits. Further, copper readily oxidizes, and so is difficult to use as a contact material without any protection, to support gallium arsenide integrated circuits.
Due to the desirability of the GaAs integrated circuit technology, there exists a need for a GaAs integrated circuit that consumes less space, is more efficiently manufactured, and uses less costly component materials.